During the hydrostatic extrusion of a billet of metallic material, the billet is surrounded by a pressure medium which exerts thereon the pressure necessary for the material of the billet to be pressed out through a passage in the die of the press in order to form a tube or rod-like product. The presses which are usually used in this connection include a pressure chamber for the pressure medium, such pressure chambers conventionally being formed within a hollow, open ended high pressure cylinder, a die arranged in one end opening of the high pressure cylinder for forming the material of the billet therethrough, and a punch insertable into the other end opening of the high pressure cylinder for generating the required pressure on the pressure medium in the pressure chamber. Internally, the high pressure cylinders are exposed to very great and extremely varying stresses, which leads to a fatigue of their portions located nearest to the pressure chamber formed therewithin. The high pressure cylinder is therefore normally constructed with an internal part, i.e., a liner which is, in the form of a tube that is exchangeable, and which can therefore be replaced by a new liner when it has been consumed by fatigue breakdown or when fatigue breakdown may be expected. In order for the liner to be as resistant as possible to the stresses, it is usually made of a high-tensile steel and is usually prestressed strongly against the surrounding parts of the rest of the high pressure cylinder.
It is an object of the present invention to considerably increase the fatigue strength of such cylinder liners, thus considerably extending their useful life. It is a further object of the present invention to provide improved liners which are useful in presses which extrude preheated billets with vegetable oils as the pressure medium.